What would cause this?

What would cause this? 2004 GEM e4. Drove about a month ago for just about 5 miles. Didn’t charge. Tried to drive today, error code 15. Noticed this. Pretty sure I turned of master power switch. Batteries at 12.4 volts.

Brass connector, steel washer, lead/tin connector, copper wire, water, solder flux. Perhaps!

clean, copper washer - LPS 3 !!